1. Field of the Invention
The present invention relates to an MRI (magnetic resonance imaging) apparatus and an MRI method which excite nuclear spins of an object magnetically with a Larmor frequency RF (radio frequency) signal and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which generate image data by single shot echo planar imaging.
2. Description of the Related Art
Magnetic Resonance imaging is an imaging method which magnetically excites nuclear spins of an object set in a static magnetic field with a Larmor frequency RF signal and reconstructs an image based on MR signals generated due to the excitation.
In the field of magnetic resonance imaging, there is an imaging method called echo planar imaging (EPI) (see, for example, Japanese Patent Application Laid-Open disclosure No. 9-276243). EPI is one of the known high-speed imaging methods in MRI. EPI is an imaging method for performing a scan which repetitively inverts a gradient magnetic field with high speed after a single nuclear magnetic excitation so as to generate a repeating train of echoes. More specifically, in EPI, all the data necessary for image reconstruction are acquired with generating continuous repetitive gradient echoes using related concurrent steps of phase encodes (PE) before NMR magnetization in the x-y plane attenuates and disappears by transverse relaxation (T2 relaxation) after applying an excitation pulse (FLIP PULSE). EPI includes SE EPI using a spin echo (SE) method to acquire spin echo signals generated after an excitation pulse and a refocus pulse (FLOP PULSE) and FE EPI with using a field echo (FE) method to acquire echo signals generating after applying an excitation pulse. While EPI generating data for a single image combined with echo train data obtained by applying an excitation pulse plural times is called multi-shot EPI, EPI to reconstruct an image by applying only a single RF excitation pulse is called single shot (SS) EPI.
FIG. 1 is a diagram showing the conventional SS SE EPI sequence.
In FIG. 1, RF denotes RF excitation pulses, ECHO denotes echo signals, Gss denotes gradient magnetic fields for SS (slice selection), Gro denotes gradient magnetic fields for RO (readout) and Gpe denotes gradient magnetic fields for phase encode respectively.
As shown in FIG. 1, in a SS SE EPI sequence, a refocus pulse is applied with a gradient magnetic field pulse for slice selection subsequently to an excitation pulse. A gradient magnetic field pulse called TUNE for adjusting a moment of the gradient magnetic field is applied in an RO direction and a PE direction respectively between the excitation pulse and the refocus pulse. In addition, after applying the refocus pulse, a spoiler gradient magnetic field pulse for the refocus pulse is applied in a SS direction.
Next, a gradient magnetic field pulse in a PE direction called BLIP pulse is applied repeatedly and a phase encode amount depending on an intensity of each BLIP pulse is added sequentially. On the other hand, a gradient magnetic field in the RO direction of which polarity inverts alternately is applied repeatedly. Consequently, echo signals necessary for generating a set of image data generates continuously and the generated echo signals are acquired. That is, echo signals for generating a set of image data can be acquired by a single nuclear magnetic excitation.
Further, as an applied technology of EPI, a diffusion weighted image (DWI) is known. A DWI is an image derived by enhancing a phase shift due to a motion of an imaging target by applying a high-intensity gradient magnetic field called MPG (motion probing gradient) pulse so as to enhance diffusion effect of the imaging target.
FIG. 2 is a diagram showing the conventional SS SE EPI sequence with applying MPG pulses for DWI.
In FIG. 2, RF denotes RF excitation pulses, ECHO denotes echo signals, Gss denotes gradient magnetic fields in the SS direction, Gro denotes gradient magnetic fields in the RO direction and Gpe denotes gradient magnetic fields in the PE direction respectively.
In the case of acquiring a DWI, for example, as shown in FIG. 2, MPG pulses are applied after applying an excitation pulse and after applying a FLOP SPOILER pulse respectively. A DWI can be acquired by performing a SS SE EPI sequence with application of the MPG pulses as mentioned above.
However, in the conventional SS EPI, there is a problem that an image may be distorted under the influence of eddy currents at some imaging positions depending on characteristics of the hardware. That is, eddy currents having spatial distributions separately to an apparatus are generated due to factors such as a type of magnet, manufacturing error or how to wind a gradient magnetic field coil. For this reason, the influence of an eddy current is different between imaging sections of which positions are mutually different spatially, and therefore, inappropriate phase encode pulses may be applied on some imaging positions. When an image is reconstructed with using data acquired under the influence of an eddy current as mentioned above, the image is distorted and the quality of the image is reduced.
In addition, since s special distribution of an eddy current changes depending on an imaging position and an application pattern of a gradient magnetic field, there is a problem that deterioration of an image quality becomes prominent at an imaging position in case of calculating an isotropic image using plural application patterns of gradient magnetic fields with applications of particularly high-intensity MPG pulses like imaging of a DWI especially.
FIG. 3 is an example of DWIs, distorted due to influence of eddy currents, acquired by performing the conventional SS SE EPI sequence.
In FIG. 3, (a) is a DWI in case of setting the b value representing a DWI intensity in the DE direction to 1000, (b) is a DWI in case of setting the b value in the RO direction to 1000 and (c) is a DWI in case of setting the b value in the SS direction to 1000.
It is confirmed that a distortion amount of a DWI acquired with a high intensity of each MPG pulse in the RO direction and the SS direction shown in FIG. 3 (b) and FIG. 3 (c) respectively is larger than that of a DWI acquired with a high intensity of a MPG pulse in the PE direction shown in FIG. 3 (a). That is, it is recognized that a different amount of distortion occurs every image section under the influence of an eddy current having a spatial distribution.
As described above, since a spatial distribution of an eddy current is not assumed in the conventional SS EPI, a different distortion occurs in an image with depending on an imaging position and an application pattern of a gradient magnetic field.